The vertices of a parallelogram are a(x1,y1), b(x2,y2), c(x3,y3), d(x4,y4)
which of the following must be true if abcd is proven to be a rectangle.
choices are attached.

For a parallelogram to be proven to be a rectange, the opposide sides must be parallel and the two adjacent sides must be perpendicular. For two parallel sides, the slope of the two sides is equal. Thus, for the parallelogram to be a rectangle, AB is parallel to CD. The slope of AB = (y2 - y1)/(x2 - x1) while the slope of CD = (y4 - y3)/(x4 - x3) Also, BC is perpedicular to CD. For two perpendicular sides, the product of the slopes is -1. The slope of BC is given by (y3 - y2)/(x3 - x2). Therefore, for the parallelogram to be a rectangle. (y2 - y1)/(x2 - x1) = (y4 - y3)/(x4 - x3) and (y4 - y3)/(x4 - x3) x (y3 - y2)/(x3 - x2) = -1. The third option is the correct answer.

The answer would be b